We heard about Tom Hiddleston dressing as Captain America on the Thor: The Dark World set when Chris Hemsworth mentioned it in an interview:

Now Empire has snagged permission to post the full deleted scene from Thor: The Dark World‘s DVD and Blu-ray bonus features ahead of its February 24th release.

As you probably already know (spoiler, we guess?) Chris Evans actually plays the Captain America part in the theatrical version of this scene. We think the alternate version exists because Hiddleston just really wanted to wear the costume. Also for fan service. God bless America.